Sitting a levels in October ?

Will I be able to sit an a level exam October 2021 if I wasn’t registered for summer exams ?

(This is for England, there are standard Autumn exam seasons in other parts of the world)

It depends. The guidance https://www.gov.uk/government/consultations/consultation-on-arrangements-for-an-autumn-2021-exam-series says:
Any student who receives a teacher-assessed grade (TAG) this summer, or who an exam board reasonably believes would have entered for the exams in summer 2021 had they taken place, can take the corresponding GCSE, AS and A level exams in autumn 2021. In addition, any student who was aged at least 16 on 31 August 2021 can take the GCSE English language and maths exams, as is usually the case.